Transport systems with a linear motor drive, i.e. so-called linear transport systems, are well known in the state of the art. The most prominent example is high-speed passenger trains on the basis of magnetic levitation technology. Transport systems with a linear motor drive, however, are also used in many industrial fields, in particular for the individual transport of piece items within production lines.
For example a linear transport system with a plurality of magnetic rotors for the transport of bottles in a container treatment facility are described in the DE 10 2013 218 389 A1. The rotors that transport the bottles thereby move while being driven by the magnetic interaction between a secondary part of the rotors that carry a permanent magnet and/or electromagnets and two long stators along two guiding rails that are guided in parallel and that are connected to the respective long stator. In this context, the rotors are installed on the guiding rails by means of rollers and generally have a chassis with a rectangular form on the level of the roller bearings, wherein roller pairs, which are only spaced at a small distance from one another, actuate on the respective guiding rail in the longitudinal direction of the rotors.
In practice, contradictory requirements arise for the design of the rotors. On one hand, the rotors should have the largest possible extension in the longitudinal direction, i.e. in the movement direction, in order to reduce wear and stress of the bearing elements, i.e. generally the rollers. In addition, the rollers can be formed smaller for a long rotor than for a short rotor. Alternatively, the rotor can take more load if the bearing elements are formed with equal dimensions.
Conversely, it is desirable to form the rotors as short as possible so that the spacing of the containers or objects transported by said rotors in the container flow that is being formed, i.e. the so-called transport spacing, is as small as possible and that consequently the throughput of containers per time unit of a container treatment facility that actuates the transport system can be as high as possible. When each rotor transports exactly one container, for example the minimum attainable spacing will arise if successive rotors drive up to contact. This minimum attainable spacing therefore corresponds to the maximum longitudinal extension of the rotors as long as the containers are smaller than the rotors. Likewise, a small longitudinal extension of the rotors is desirable in order to be able to take up small containers from a transport belt at a transition point in the accumulated state.
In determining the maximum attainable transport spacing, the extension of the chassis of the rotors is generally the limiting factor as said chassis determines the position and/or the spacing of the bearing elements, for example of the rollers. In addition, however, also the extension of the secondary part in the transport direction, which usually has a carrier plate with magnets installed on it, can restrict a reduction of the minimum attainable transport spacing. For example, the extension of the magnets in the transport direction is predetermined by the design of the coils of the long stator and can therefore not be reduced at random, wherein usually a sequence of multiple magnets that are polarized in an alternating way is provided in the movement direction, i.e. longitudinal direction, of the rotors in order to achieve an optimal propulsion force. The longitudinal extension of this sequence thereby defines the maximum realizable longitudinal extension of the secondary parts and therefore sets a lower limit for the maximum attainable transport spacing.
In practice, the maximum attainable transport spacing is even clearly greater than this longitudinal extension of the secondary parts because the secondary parts of rotors that succeed one another directly would, depending on the polarity of the magnets of the adjacent sequence ends, either attract or reject each other strongly in case of a too close approximation. In both cases, the arising forces would exceed the propulsion force of the linear motor by far so that a controlled movement of the rotors would no longer be possible. For this reason, a sufficiently large gap will always remain between the secondary parts of successive rotors in practice.